calm
Meteorol of force 0 on the Beaufort scale; without wind
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005
calm
[käm] (meteorology)
The absence of apparent motion of the air; in the Beaufort wind scale, smoke is observed to rise vertically, or the surface of the sea is smooth and mirrorlike; in U.S. weather observing practice, the wind has a speed under 1 mile per hour or 1 knot (1.6 kilometers per hour).
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
calm
The absence of wind or an apparent lack of motion of the air. It is depicted on synoptic charts as 
.
.An Illustrated Dictionary of Aviation Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.
Calm
a condition of little or no wind. On the Beaufort scale, wind velocity during a calm is 0–0.2 m/sec. Calms are usually observed within stable anticyclones, in cols, and in the intertropical convergence zone. Calms occur more often in valleys and basins than in open areas with free circulation of air. There is little turbulence during a calm, and therefore harmful atmospheric aerosols (smoke and gas) may accumulate in the lowest atmospheric layer.
The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.